Transcript list - Duke University

Project JETT
Java Engagement for Teacher Training
Duke University Department of Computer Science
Association for Computing Machinery
College Board
Project JETT/Duke
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Goals of JETT

ACM's long-term goal is the creation of a community of
computer science teachers, university faculty, undergraduate
and graduate students, as well as College Board Lead
Teachers.

The short-term objective is to assist high school computer
science teachers in making the AP language switch from C++
to Java for the upcoming academic year.

ACM is partnering with the College Board and selected
universities to provide teachers with on-site workshops as
well as remote training via the ACM K-12 Web Repository.
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Java primitives (not objects)


Logical :
boolean
 No boolean/int conversion or cast
while(list) {…}
Text:
char
 Unicode, not part of AP subset

Integral:
byte, short, int, long
 By definition: 8, 16, 32, 64 bits long

Floating point:
double, float
 Use double, float not in the subset
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Java Object Basics

Class is an object factory
 Class can have (static) methods
 Defines interface programmers depend on
 Part of inheritance hierarchy (every class is-an Object)
 May define getter and setter methods for accessing state
 Usually defines methods for behavior

An object is an instance of a class
 Attributes aka state, usually private, sometimes accessible
 Behavior invoked by calling methods
 Allocated by calling new
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When are objects equal?

For primitive types we test for equality with ==
 We expect 7 == 7, and x == y if both are 7

For objects the == check sees if addresses are the same
 Remember for x == y non-primitive, pointers used!

All classes inherit equals(..) from Object, must override in
subclasses
public boolean equals(Object o)
{
SimpleRectangle s = (SimpleRectangle) o;
return width == s.width && height == s.height;
}
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Java Array basics

Syntactically similar to C++ arrays
int[] list = new int[10];
// values stored?
String list2[] = new String[100];
for(int k=0; k < list.length; k++) {
list[k] = 3;
}
Arrays.fill(list2,"Hello"); // not in subset

Once created cannot resize, indexing is range-checked
 Array object is-an Object via inheritance
 Default value is 0/null, not in subset
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ArrayList basics

ArrayList stores Objects, not int, not boolean, yes String …
 Don't have syntactic sugar of [] indexing
 Must cast when getting object out of ArrayList
• Except when treating as an object, e.g., using toString()

Can add to end in constant time, in middle with shifting
• Array grows, use size() method to determine # elements
ArrayList list = new ArrayList();
list.add(new String("hello"));
list.add(new String("world"));
String s = (String) list.get(0); // what is it?
list.set(0, new String("big"));
list.add(0, new String("great"));
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Arrays and the AP subset

One and two-dimensional arrays in subset
 Two-dimensional arrays will move to AB only
int[][] grid = new int[6][10];
int rows = int.length;
int cols = int[0].length;

Initialization in subset, e.g., int[] list = {1,2,3,4,5};

No java.util.Arrays methods in subset
 sort, binarySearch, fill, asList, …
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ArrayList and the AP subset

Inheritance hiearchy (List in java.util) is AB only
 Iterator and ListIterator are AB only

Downcast from Object to expected type is in subset
list.add(new String(“hello”));
String s = (String) list.get(0);
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Required methods :
 size(), get(int), set(int, Object),
 add(Object), add(int, Object), remove(int)
NOT required:
 remove(Object), addAll(Collection), clear()
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What is an Iterator?

What problems do Iterators address?
 Access elements independently of implementation
 Client programs written in terms of generic component
public void print(Collection c)
{
Iterator it = c.iterator();
while (it.hasNext()) {
System.out.println(it.next());
}
}

How do you add all elements of Set to a List?
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What is an interface?

Indication to programmers and code that a class implements
some specified functions, most likely with requirements on
behavior
 Iterator is an interface: what do we expect from classes that
implement the Iterator interface?
 Comparable: are some objects incomparable? Why?
 Why isn’t Equatable an interface? Where is .equals() ?

A class can implement multiple interfaces
 Comparable, Cloneable, Tickable, …
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Think twice before developing inheritance hierarchy
 Single inheritance, problems with protected/private data
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What is Comparable?
String a = “hello”;
String b = “zebra”;
int x = a.compareTo(b);
int y = b.compareTo(a);
int z = a.compareTo(a);
// what values assigned?

Contract: compareTo() should be consistent with equals()
 What’s the simple way to write equals for Comparable?
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See also java.util.Comparator
 Not in subset, useful for sorting on other criteria
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Guidelines for using inheritance

Create a base/super/parent class that specifies the behavior
that will be implemented in subclasses
 Some functions in base class may be abstract
• Subclasses required to implement, or cannot create object


Consider using an interface if there’s no default behavior
or state to provide
Inheritance models “is-a” relationship, a subclass is-a parentclass, can be used-as-a, is substitutable-for
 Standard examples include animals and shapes
• Square should NOT derive/inherit from rectangle
• A square is NOT a rectangle in programming terms
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Inheritance (language independent)

First view: exploit common interfaces in programming
 Streams in C++, Iterator in Java
• Iterators in STL/C++ share interface by convention/templates

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Implementation varies while interface stays the same
Second view: share code, factor code into parent class
 Code in parent class shared by subclasses
 Subclasses can override inherited method
• Can subclasses override and call?

Polymorphism/late(runtime) binding (compare: static)
 Actual function called determined when program runs, not
when program is compiled
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Top 10: Choosing array vs ArrayList
10.
a.length has same number of characters as a.size()
but doesn’t require using the shift key
9.
Too many options: int[] list compared to int list[]
8.
Array initialization with int[] list = {1,2,3,4,5};
7.
Freedom not to choose:
for(int k=0; k < a.size(); k++) …
Iterator it = a.iterator(); while (it.hasNext())…
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Top-10: choosing array vs ArrayList
6.
ArrayList, what’s that: a list or an array?
5.
“Take away my Integer, but leave me my int” --- Puff Daddy
4.
You can add a String to an ArrayList, but you can’t get a
String out of an ArrayList (well, ok, you can with a cast).
3.
Collections.sort is stable, Arrays.sort isn’t (mostly)
2.
list[k] = list[j] vs. list.set(k,list.get(k));
1.
No method to shuffle an array (Collections.shuffle)
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